The general goal of the work in progress on this project is to understand the specific ways in which macromolecules recognize and combine with other macromolecules. This knowledge is fundamental to understanding the high specificity and fidelity of the biochemical processes occurring in organisms. Since these processes are largely a manifestation of structural features of the relevant biological macromolecules, we are studying the structures of several such molecules by X-ray diffraction methods and seeking to show how these structures determine the specificity of the processes in which they are involved. The two systems which we are studying most intensively are the agglutination of cells by wheat germ agglutinin and the recognition of tRNA by the amino-acyl-tRNA-ligases. In the former, we have determined the structure of the wheat germ agglutinin molecule and are currently studying its complexes with small polysaccharides as well as large glycoproteins. We have determined the structure of yeast tRNA Gly and are comparing it with that of yeast tRNA Phe. The differences which we observe in these two structures are being examined for their potential significance in discriminating among tRNA molecules in their reactions with synthetases.